text: Four-wave mixing is a process where intense light at three frequencies  interact to produce light at a fourth frequency . If the three frequencies are part of a perfectly spaced frequency comb, then the fourth frequency is mathematically required to be part of the same comb as well.
  Starting with intense light at two or more equally spaced frequencies, this process can generate light at more and more different equally spaced frequencies. For example, if there are a lot of photons at two frequencies , four-wave mixing could generate light at the new frequency . This new frequency would get gradually more intense, and light can subsequently cascade to more and more new frequencies on the same comb.
  Therefore, a conceptually simple way to make an optical frequency comb is to take two high-power lasers of slightly different frequency and shine them simultaneously through a photonic-crystal fiber. This creates a frequency comb by four-wave mixing as described above.